bswapu64.c

typedef unsigned char u8; 

static inline uint64_t bswapu64(uint64_t a)
{
    uint64_t b = 0;
    for (u8 i = 0; i < sizeof(uint64_t); ++i) {
        b = (b << __CHAR_BIT__) | (u8)a;
        a >>= __CHAR_BIT__;
    }   
    return b;
}